[xsl] Sum of values from XML

Given the following XML structure:

<a>
  <b>
    <c>23</c>
    <d>4</d>
  </b>
  <b>
    <c>45</c>
    <d>67</d>
  </b>
</a>

Using XSLT 2 how can I retreive the sum of all the integers at /a/b/c?

thanks

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